1. Field of the Invention
The invention relates generally to the exploitation of hydrocarbon-containing formations. More specifically, the invention relates to clean up of non-aqueous based well fluids.
2. Background Art
When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons For purposes of description of the background of the invention and of the invention itself, such fluids will be referred to as xe2x80x9cwell fluids.xe2x80x9d Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroleum bearing formation), transportation of xe2x80x9ccuttingsxe2x80x9d (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, implacing a packer fluid, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
As suggested above, a variety of potential well fluids are available for use, depending on the particular application. One significant class of well fluids is so called xe2x80x9coil-basedxe2x80x9d well fluids. Depending on the application, various hydrocarbons or other non-aqueous fluids may be used as a major component of the well fluid. For example, diesel fuel is commonly used as a principal component of certain well fluids. However, if the environment in the vicinity of the wellbore is particularly sensitive, or is subject to regulations on the use of diesel fuel as a well fluid component, solvent grade minerals oils and organic liquids including esters, paraffins, acetals, glycols, and olefins may be used. When these fluids are manufactured from lower molecular weight components, they may be referred to as synthetic, and their well fluids referred to as xe2x80x9csynthetic-based.xe2x80x9d Both diesel and these latter fluids are included in the term xe2x80x9cnon-aqueous.xe2x80x9d
Throughout the specification, the phrase xe2x80x9coil-in-waterxe2x80x9d is used interchangeably with xe2x80x9cnon-aqueous.xe2x80x9d The phrase xe2x80x9coil-in-waterxe2x80x9d is intended to encompass all non-aqueous-in-water systems, and the term oil is used instead of non-aqueous for convenience only, and is not intended to limit the scope of the invention in any fashion. In most non-aqueous fluids, a separate aqueous phase is emulsified into the non-aqueous fluid to provide selected properties desirable for drilling. Generally the aqueous phase is a brine. Because emulsification of oil into water was developed first, these water-in-oil emulsions are often referred to as xe2x80x9cinvert emulsions.xe2x80x9d Typically, if a non-aqueous fluid is used in conjunction with brine, a phase separation between the oil and brine occurs. To secure a stable emulsion, a surfactant capable of emulsifying water into the hydrocarbon is an essential component of non-aqueous well fluids. One particular class of non-aqueous fluids are those which are xe2x80x9creversiblexe2x80x9d (i.e., the emulsion can be converted from a water-in-oil to an oil-in-water emulsion) by the action of a chemical or physical trigger such as pH, oxidizers, heat, light, microwaves, etc.
Non-aqueous fluids are often chosen for use in drilling fluids because of their excellent lubrication properties, their chemical stability, and their minimal chemical impact on certain earth formations that are sensitive to water-based well fluids. These properties permit the drilling of wells having a significant deviation of the well bore trajectory from vertical, as is typical of offshore or deep water drilling operations where highly inclined or even horizontal wells may be drilled. For example, in highly inclined wells, torque and drag on a drilling tool assembly used to drill the well (the xe2x80x9cdrillstringxe2x80x9d) are significant because the drillstring typically lies against the bottom of the well (the xe2x80x9clow side of the holexe2x80x9d), and the risk of the drillstring becoming stuck in the well is high. Use of non-aqueous fluids has been shown to significantly reduce drill string sticking and similar problems encountered during directional drilling.
However, there are some disadvantages of using non-aqueous well fluids. Most notably, the disposal of formation cuttings removed from a well drilled with non-aqueous well fluids is a primary concern, especially for offshore or deep-water drilling operations or particularly remote and sensitive land-based operations. In many cases, environmental regulations require use of expensive synthetic fluids and the cuttings must meet stringent requirements for chemical content before they can be discharged into the environment.
To meet environmental discharge requirements, the cuttings generally must either be washed clean of the oil before disposal at sea or be injected as a slurry into an earth formation or be disposed on land in an environmentally safe manner.
U.S. Pat. No. 6,218,342 (the ""342 patent), issued to Patel, and assigned to the assignee of the present invention, describes a class of suitable surfactants for use in xe2x80x9creversiblexe2x80x9d invert emulsion fluids. In particular, the ""342 patent describes the use of an amine surfactant, which, upon protonation, causes the invert emulsion to xe2x80x9cflipxe2x80x9d and become a regular emulsion. In order to function, the amine surfactant described in the ""342 patent requires that a protonating agent (an acid, for example) be added to the invert emulsion. After a sufficient amount of protonating agent, cited in the patent as mineral and organic acids or systems that generate such acids, is added, the amine surfactant becomes protonated and the emulsion flips from an invert emulsion to a regular emulsion.
The ""342 patent also describes a purification method which includes separating oil-laden cuttings using conventional solids removal methods (such as sift-screening). The oil-laden cuttings are then contacted at least once with an acid solution, again defined as mineral and organic acids, so as to invert the emulsion coating the cuttings. Washing with acid causes the cuttings to convert from being xe2x80x9coil-wetxe2x80x9d to being xe2x80x9cwater-wetxe2x80x9d solids, allowing the substantial removal of the oil from the cuttings. In order to cause the xe2x80x9cflipxe2x80x9d from invert to regular emulsion, the pH of the system is typically lowered to about 2.
A significant, and problematic, side reaction occurs at this low pH. Generally speaking, when well fluids having a pH of greater than about 5.5 (which is typical) are passed through the air, they absorb carbon dioxide. When lime is present in the cuttings (which is often the case in cuttings drawn from the wellbore), the carbon dioxide and lime react to form calcium carbonate. When the pH is lowered below about 5.5, the absorbed carbon dioxide is evolved, resulting in the formation of gas bubbles at the point of acid contact. These gas bubbles very often form stiff foams in the presence of the other chemicals present in the well fluid, causing significant clean-up problems. Because calcium carbonate is also one of the main binders of sandstones, its dissolution can cause nominally acid insoluble rock to decrepitate into fine particles that are difficult to separate from wash fluids. What is needed are methods and compositions for use with invert emulsion well fluids that effectively remove the drilling fluids without decrepitating the rocks or yielding foam-producing carbon dioxide.
In one aspect, the present invention relates to a method for cleaning cuttings arising from the use of an invert emulsion well fluid, which comprises contacting the cuttings at least one time with a sufficient amount of a buffered wash.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.